It’s no secret that Carrie Underwood is serious about—and seriously committed to—her fitness. Her first public appearance after breaking her wrist in a nasty fall last month? The gym, naturally, where Underwood was spotted last week by reality TV star Adrienne Gang. Underwood was sporting a wrist brace, so while she may not be back to her full push-up sets just yet, there is one area she’s most definitely working: her strong, powerful legs.

Now, thanks to her in-home trainer, Erin Oprea, whose roster of celeb clients includes Kelsea Ballerini, Martina McBride, and Jennifer Nettles, we know one of the magic moves that keeps Underwood’s lower limbs in such great shape. It’s a two-part series of lunge variations that work essentially every muscle in the bottom half of the body. “It’s so effective for your booty, back of the legs, and quads,” Oprea tells SELF. “It will put your quads on fire!”

Oprea, who trains Underwood two to three times a week, says she’s been using this two-part leg exercise in workouts for several years now. It’s especially challenging because it involves three key tweaks to the classic lunge: a forward fold of the chest, intense pulsing, and small lifts of the back leg.

Here's how to do the first part of the lunge series:

Stand with your feet hip-width apart. Step your right foot forward and lower into a deep lunge position, with your left knee hovering one to two inches above the ground.

Why is this variation so effective? For one, bringing your chest forward puts more work on your glutes. “It fires off your glutes a lot,” Oprea says. “It’s a drastic difference from upright standing [lunges].” The pulsing helps build endurance in your quads, glutes, and hamstrings. And lifting up your back leg forces you to put all of your weight on your front leg. “This isolates all of the muscles in that front leg and doubles the burn. It’s like doing a one-legged lunge,” she says.

This move is similar to the first lunge, except the curtsy squat position puts more focus on the outside of your butt and hips. It specifically targets the hip abductor muscles (the muscles that move your legs outward away from your body) and the stabilizer muscles that keep your legs and hips in proper alignment.

And here's how you can combine them into a quick legs workout.

These lunges are great to incorporate into your regular workout, but you can also combine them into a quick bodyweight legs circuit. Here's what Oprea suggests for a quick legs workout:

Do the first lunge variation for 20 seconds, then rest 10 seconds.

Do the second lunge variation for 20 seconds, then rest 10 seconds.

Repeat this 8 times.

This type of routine is known as a Tabata workout, which involves 20 seconds of high intensity effort followed by 10 seconds of rest, repeated 8 times. (The original Tabata protocol was done on special exercise bikes, but since then, trainers have adapted the method to refer to pretty much any workout that combines short high-intensity intervals with even shorter rest intervals.) “I use Tabata with so many of my clients,” Oprea says, “because it makes the workout feel more like a game but it’s still so effective." Because you're going so hard during the high-intensity portions, you're fitting in a lot of work in a very short amount of time.

Tip: Be sure to engage your core as you pulse up and down. It’s important to keep your stomach muscles tight as you perform the moves, Oprea says, as that will aid your balance. “Your core is your stability,” she explains.

If you’re still experiencing trouble balancing or feel wobbly in the deep lunge position, Oprea suggests using the hand of the leg that is forward to grab a chair or place it against the wall. If you’re looking to make it harder, you can do these moves holding a dumbbell in the hand of the leg that is back (e.g. if your left leg is back, hold the dumbbell in your left hand). “But it’s important to master good form first,” Oprea advises.